Pulmonary veins and normal variant
Pulmonary veins (PV) are the terminal vascular structure of the pulmonary circulation that returns oxygenated blood from the lungs back to the left atrium. Anatomy Formation There are normally four PVs, formed from convergence of lobar veins and segmental veins from the lung. These are the right superior pulmonary vein (RSPV), right inferior pulmonary vein (RIPV), left superior pulmonary vein (LSPV) and left inferior pulmonary vein (LIPV)(1). The RSPV, mean length 15mm, is formed by the apical, anterior and posterior segmental veins that drain the right upper lobe, and right middle lobar vein (RMLV) that drains the right middle lobe. The RIPV, mean length 7mm, is formed by the superior and common basal veins that drain the right lower lobe. On the left side, the LSPV, mean length 18mm, is formed by the apicoposterior, anterior and lingular veins that drain the left upper lobe. Finally the LIPV, mean length 14mm, is formed by the superior and common basal veins that drain the left lower lobe (1, 2). Relations and features: The four PVs pass medially toward the heart through the middle mediastinum from their hilar origins. They pierce through the fibrous pericardium and terminate in the posterior aspect of the left atrium as four venous ostia. Their termination sites are separated centrally by the oblique pericardial sinus. The superior veins are usually more oblique and lie anteroinferior to the pulmonary artery. Course of the inferior PV are essentially horizontal and forward toward the heart. They lie posterior to the superior PV and is the most inferior structure of the hilum. Superior PVs are larger and longer than inferior PVs (1, 2). The right PV travel behind the superior vena cava and enters the most lateral and superior portion of the left atrium. The left PV passes in front of the descending aorta and join the left atrium near the atrial appendage (1, 2). Normal variations The literature reports a prevalence of 20-50% of population having normal deviation of PV anatomy (2, 5, 7-10). Variations are broadly categorized into common pulmonary trunks or accessory pulmonary veins. Understanding these variations bear important clinical relevance, as multiple accessory ostia due to supernumerary PV may increase risk of atrial arrhythmias and affect success rate of ablative therapies (2, 5, 6, 8, 10). Common pulmonary trunk Prior to reaching the heart, the superior and inferior PVs unite to form a larger common pulmonary trunk which enters the left atrium via a common single vessel ostia. This is because of an under-incorporation of PV into the LA, thus preserving the common pulmonary trunk (5, 6). This variant is more common on the left than the right (2, 5, 7-10). Accessory pulmonary vein These are due to over-incorporation of PV beyond its first division and involving the lobar and segmental veins. These lobar and segmental veins become accessories veins that run together with the PV but enter the left atrium directly with their own independent ostia. Accessory veins are much smaller than PVs(2). The most common are the middle lobar accessory vein, followed by right segmental accessory veins (superior and basal segmental of the right lower lobe) (2, 8, 10). The middle lobar vein is normally formed by a lateral and medial venous component. If PV incorporation proceeds beyond this division, two RMLV accessories are observed (8). Left PV accessory veins are exceedingly rare and only a few cases have been reported in literature involving, for example, the lingular vein (10). Other variations Aside from variations in numbers of PV, less commonly, the branching pattern can also differ. In the example of RMLV, up to 15% of cases, this vein drains into the RIPV instead of the RSPV(10). Other even rarer examples reported in literature include the superior segmental of the right lower lobe draining into the RSPV instead of RIPV, or vice versa the apical segmental vein of the right upper lobe draining into the RIPV instead of the RSPV(10).